Method of applying a luminescent layer to the hollow side of a support having a rotation-symmetrical curved surface



Aug. 28,1956 M. c. TEVES ETAL 2,750,832

METHOD OF APPLYING A LUMINESCENT LAYER TO THE HOLLOW SIDE OF A SUPPORT HAVING A ROTATION-SYMMETRICAL CURVED SURFACE Filed June 6, 1952 4 Moral? i INVENTORS. Mmmv (anvil/s Yin 1:5 y 221m JbL AGF T United States Patent METHOD OF APPLYING A LUMINESCENT LAYER TO THE HOLLOW SIDE OF A SUPPORT HAVING fifigETATION-SYMMETRICAL CURVED SUR- Marten Cornelis Teves and Taek e Tol, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application June 6, 1952, Serial No. 292,202 Claims priority, application Netherlands June 20, 1951 1 Claim. (Cl. 117-335) This invention relates to methods of applying a luminescent layer to the hollow side of a support having a rotation-symmetrical curved surface.

It is known that with electron tubes for converting a first image into a second luminescent image with the use of an electron-optical reproduction system, the lens fields for the electron-optical image reproduction may be obtained in a simple manner, if the surface of the photoelectric cathode has a spherical shape. In such a tube opposite the concave surface of the photocathode, only an acceleration anode having a small aperture to allow the electrons to pass need be arranged to obtain a second image corresponding satisfactorily with the first image on the photocathode, on the flat collecting screen arranged beyond the aperture of the, anode. The similarity of the first image to the second image may be further improved if the anode has a rounded top facing the photocathode.

For the use of such devices in the examination of patients by means of X-rays the photocathode collects light radiated by a fluorescent screen due to the X-rays. The photo-electric sensitive layer is preferably spread over the luminescent layer of the screen, so that no optical projection means are required to reproduce the light image on the photo-cathode.

For the manufacture of flat fluorescent screens a quantity of the luminescent material is mixed with a suitable adhesive and spread in uniform thickness over a support with the use of a spreading tool.

When using a spherically curved photo-electric cathode, which is in contact throughout its surface with the luminescent layer, first a fluorescent screen must be made, which has the required curvature. The uniform distribution of the suspension over the hollow surface of a curved support cannot be obtained by spreading and gives rise to considerable difiiculties. v

With known methods used for applying luminescent layers in the manufacture of cathode-ray tubes for reproducing television images, a thin liquid suspension of luminescent material in a liquid is poured into the tube over the support of the luminescent layer. This is mostly the bottom of the tube, which may be flat, but which may also be slightly spherically curved in order to have a greater resistance to the pressure of the atmosphere. After the suspension has been poured into the tube, the luminescent substance is allowed to settle, then the liquid is decanted carefully out of the tube. If, when pouring the liquid away, the support is excessively tilted, the settled solid may be disturbed so that thick andthin spots are formed and the screen is spoiled. This phenomenon, which may occur if the support is tilted by as little as 5, varies with the granular size of the luminescent substance and with the nature of the material from which the support is made. A tilting greater than 15 is inadmissible.

A further disadvantage of these known methods of applying a luminescent layer to a concave surface is that the distance in a vertical direction of the support from 2,160,882 Patented Aug. 28, 1956 IQE the horizontal liquid level is not constant. The quantity of luminescent substance over equal surface areas of the support difiers and therefore the thickness of the settled solid decreases from the centre of the surface towards the edge.

Where the surface of the support has a slope with respect to the liquid level exceeding the aforesaid values of 5 to 15, the deposit may shift towards the flatter part of the support to accumulate therein greater quantity. A layer of uniform thickness cannot be readily obtained in this manner.

The invention will be more fully understood from the accompanying drawing which is a view partly in elevation and partly in section of an apparatus used to carry out the method of the invention.

According to the invention, a method of applying a luminescent layer to the hollow side of a curved support having a rotation-symmetrical surface with the use of a suitable adhesive, of which a greater quantity than is required for securing the luminescent material to the support is mixed with the luminescent substance to form a suspension, is characterized inthat a quantity of the suspension is applied to part of the support and is distributed in a layer of substantially uniform thickness over the surface of the support by rapid rotation of the support about its axis of symmetry.

The speed of rotation is chosen to be such that the resultant of the force of gravity acting upon the particles of the luminescent substance and the centrifugal force acting upon the particles is approximately at right angles to the surface of the support at any point on the support. At this speed, the luminescent substance may settle on the surface to form a layer of substantially uniform thickness. After settling, the luminescent substance forms an uninterrupted layer, of which the resistance to disturbance has already considerably increased. The solidity of the layer may be increased by partial drying during the settling, for example, by heating the support. The excess liquid may then be removed by increasing the speed of rotation such that some of the adhesive spills away across the edge of the support, the settled luminescent material,

1 however, remaining in position.

One example of the method according to the invention will now be given with reference to the drawing, the method being applied to the manufacture of a fluorescent screen for X-ray image intensification.

The binder used is a silicone, product of the Dow Coming Corp., Midland, Michigan, United States of America, designated in the catalogue as Silicone 804."

The hollow side of a thin spherical aluminium plate 1, for example, 160 mms. in diameter, of which the radius of curvature is about 20 cms., serves as a support for the luminescent substance and is secured to a horizontal rotating disc 2 in a manner such that the axis of symmetry of the support coincides with the axis of rotation of the rotary disc.

Near the centre of the aluminium plate a quantity of a suspension of a blue luminescent phosphor in silicone is provided. The suspension may be formed by mixing 240 gms. of phosphor with ccs. of silicone. Of this mixture, for example, 12 ccs. is used.

After having poured in the liquid, the disc is'rotated by shaft 3 and the speed is controlled to be such that the suspension flows uniformally over the surface to about 1 cm. from the edge. The rotation of the rotary disc is continued uninterruptedly and at constant speed until the luminescent material has settled from the suspension. The time required for this operation is about 10 to 20 minutes. During this time a drying process may be initiated, for example, by heating the support. Thus the possibility of movement of the phosphor particles which 3 settle in an already comparatively solid layer is further reduced; this has the advantage that upon a variation of the speed of rotation the layer loses its cohesion les quickly.

A further advantage-is obtained, since with slight heating the silicone over the phosphor layer becomes less viscous. A favourable heating temperature lies between 45' C. and 75 C. In order to drain the excess quantity of adhesive, the rotation speed is increased, after the phosphor has settled such that the adhesive moves farther up the support and spills away across the edge of the support. The time required for this operation is shorter as the adhesive liquid decreases in viscosity. Consequently, the duration of the process is shortened by heating.

The quantity of adhesive remaining on the layer of luminescent material may be controlled by the adjustment of the rotation speed. This speed may be chosen to be such that substantially not more silicone remains than is required for a satisfactory adhesion of the grains of the luminescent material. Thus the surface becomes slightly uneven, since the spaces between the grains are not completely filled.

It is better to adjust the rotation speed to be such that the silicone remaining on the screen still forms a cohesive layer just completely covering the points of the phosphor grains. In this case the screen has a dull shiny surface and an optimum optical contact is obtained between the luminescent layer and the photo-electric cathode to be this temperature, the silicone hardens completely in about 16 hours.

forming a liquid suspension of luminescent particles in a silicone binder, depositing a quantity of said suspension on a central portion of said support, rotating said support about said central axis of symmetry at a speed at which the resultant of the force of gravity and the centrifugal force acting on the luminescent particles is approximately at right angles to said surface of said support, heating said support while it is rotating to about 40 C. to 75 C. until the silicone binder becomes less viscous, increasing the speed of rotation of said support to remove the excess binder by spilling the same across the edge of the support,

applied afterwards. In this form, the resolving power and the image sharpness are maximal.

After the speed of rotation for obtaining the desired separation is correctly adjusted, the temperature is raised for the further working of the screen until the layer is dry and the silicone begins to polymerise. After two or three hours, the rotation may be stopped, after which the screen is heated to a mperature of about 200 C. At

and slowly increasing the heating of said support to effect hardening of said remaining binder.

References Cited in the file of this patent UNITED STATES PATENTS 2,527,134 Johnson Oct. 24, 1950 2,619,430 Fink Nov. 25, 1952 2,644,770 Sadowsky July 7, 1953 FOREIGN PATENTS 587,741 Great Britain May 5, 1947 671,924 Great Britain May 14, 1952 

